With the remarkable development of information technology (IT), a great variety of portable information communication devices has been popularized. As a result, in the 21st century, we are moving toward a society based on ubiquitous access, in which high-quality information service is possible regardless of time or place.
Lithium secondary batteries are very important in realizing such ubiquity. Specifically, lithium secondary batteries, which can be charged and discharged, have been widely used as energy sources for wireless mobile devices. In addition, lithium secondary batteries have also been used as energy sources for electric vehicles and hybrid electric vehicles, which have been proposed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
As devices to which the lithium secondary batteries are applicable are becoming more diverse, as described above, lithium secondary batteries have also been diversified such that the lithium secondary batteries can provide outputs and capacities suitable for the devices to which the lithium secondary batteries are applied. In addition, there is a strong need to reduce the size and weight of lithium secondary batteries.
Based on the shape thereof, the lithium secondary batteries may be classified into a cylindrical battery cell, a prismatic battery cell, and a pouch-shaped battery cell. Among these kinds of lithium secondary batteries, much interest is currently focused on the pouch-shaped battery cell, which can be stacked with high integration, has high energy density per unit weight, can be manufactured at low cost, and can be easily modified.
The pouch-shaped battery cell is configured to have a structure in which an electrode assembly having a positive electrode/separator/negative electrode structure is contained in a battery case made of a laminate sheet together with an electrolyte in the state in which the outer edge of the battery case is sealed by thermal fusion.
FIG. 1 schematically shows the general structure of a representative pouch-shaped battery cell 10 including a stacked type electrode assembly.
Referring to FIG. 1, the pouch-shaped lithium secondary battery 10 includes an electrode assembly 30, electrode tabs 40 and 50 extending from the electrode assembly 30, electrode leads 60 and 70 connected respectively to the electrode tabs 40 and 50 by welding, and a battery case 20 for receiving the electrode assembly 30.
The electrode assembly 30 is a power generating element including positive electrodes and negative electrodes sequentially stacked in the state in which separators are interposed respectively between the positive electrodes and the negative electrodes. The electrode assembly 30 is configured to have a stacked type structure or a stacked/folded type structure. The electrode tabs 40 and 50 extend from corresponding electrode plates of the electrode assembly 30. The electrode leads 60 and 70 are electrically connected to the electrode tabs 40 and 50, extending from the corresponding electrode plates of the electrode assembly 30, respectively, for example, by welding. The electrode leads 60 and 70 are partially exposed outward from the battery case 20. In addition, insulative films 80 for improving a seal between the battery case 20 and the electrode leads 60 and 70 and, at the same time, securing electrical insulation between the battery case 20 and the electrode leads 60 and 70 are attached to portions of the upper and lower surfaces of the electrode leads 60 and 70.
The battery case 20 is made of an aluminum laminate sheet. The battery case 20 has a space defined therein to receive the electrode assembly 30. The battery case 20 is generally formed in the shape of a pouch. In the case in which the electrode assembly 30 is a stacked type electrode assembly as shown in FIG. 1, the inner upper end of the battery case 20 is spaced apart from the electrode assembly 30 such that the positive electrode tabs 40 and the negative electrode tabs 50 can be coupled to the electrode leads 60 and 70, respectively.
Each of the electrode tabs of the pouch-shaped secondary battery is made of foil having a thickness of 0.5 mm or less. The electrode tabs of the pouch-shaped secondary battery are electrically connected to the electrode leads and are coupled to each other by welding.
When gas is continuously generated in the conventional pouch-shaped battery cell having the above structure due to overcurrent and overvoltage in the battery cell during the charge and discharge of the battery cell, however, the pressure of the gas in the battery cell excessively increases. As a result, a portion of the sealed region of the battery case is open, i.e. a vent is formed in the battery case, whereby the gas is discharged from the battery case. Even in this state, charging and discharging current continuously flows in the electrode assembly, since the electrode assembly, in which current flows, is not affected. As a result, gas is continuously generated in the battery cell, and the gas is discharged out of the battery cell. The discharged gas is harmful to the human body. In addition, the temperature of the battery cell increases, which may result in the ignition and explosion of the battery cell.
The conventional pouch-shaped battery cell is configured to include an additional system disposed outside the battery cell for preventing gas from being generated in the battery cell and preventing further reaction from taking place in the battery cell when the battery cell is saturated. When the external system malfunctions, however, abnormal charge and discharge are continuously performed in the battery cell, whereby the performance and safety of the battery cell are lowered.
Therefore, there is a strong necessity for a battery call that is capable of solving the above problems.